The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Internal combustion engines combine air and fuel to create an air/fuel (A/F) mixture that is combusted within a plurality of cylinders. The combustion of the A/F mixture drives pistons which rotatably turn a crankshaft generating drive torque. Specifically, air may be drawn into the cylinders and compressed using the pistons. Fuel may then be combined with (i.e., injected into) the compressed air causing the pressurized A/F mixture to combust (e.g., a compression ignition, or CI engine). For example, CI engines include diesel engines.
Alternatively, the air may be mixed with fuel to create the A/F mixture prior to compression. The A/F mixture may then be compressed until the A/F mixture reaches a critical pressure and/or temperature and automatically ignites (e.g., a homogeneous charge compression ignition, or HCCI engine). HCCI engines, however, may also “assist” ignition of the A/F mixture using spark from spark plugs. In other words, HCCI engines may ignite the A/F mixture using spark assist depending on engine operating conditions. For example, HCCI engines may use spark assist at low engine loads.
Exhaust gas produced during combustion may be expelled from the cylinders into an exhaust manifold. The exhaust gas may include carbon monoxide (CO) and hydrocarbons (HC). The exhaust gas may also include nitrogen oxides (NOx) due to the higher combustion temperatures of CI engines and HCCI engines compared to spark ignition (SI) engines. An exhaust treatment system may treat the exhaust gas to remove CO, HC, and/or NOx. For example, the exhaust treatment system may include, but is not limited to, at least one of an oxidation catalyst (OC), a particulate matter (PM) filter, a selective catalytic reduction (SCR) system, NOx absorbers/adsorbers, and catalytic converters.